Vacuum device



United States Patent I I 3,330,472 VACUUM DEVICE Joseph C. Maliakal,Newton, Mass., assignor to National Research Corporation, Cambridge,Mass., a corporation of Massachusetts Filed Oct. 1, 1965, Ser. No.492,057 Claims. (Cl. 230-69) The present invention relates to electricaldevices of the orbitron type and particularly to orbitron pumps.

Orbitrons are Well known from the work of the University of Wisconsin.These devices provide an orbiting cloud of electrons which ionize gas inan enclosed annular pumping region through electron-molecule collisions.

I have developed an improved mode 'of construction of such devices whichallows increased mean free paths of electrons for higher ionizationefiiciency, operation at higher currents if desired, and simplifies theconstruction of the filament and filament supports of the electronsources for orbitron devices. These features afford the importanteconomic and technical advantages of higher inert gas pumping speeds inorbitron pumps and reduced cost of construction and operation of allorbitron devices utilizing my invention.

It is therefore the object of the invention to provide an improvedorbitron device aifording the above features and advantages.

Other objects, features or advantages of the invention will, in part, beobvious and will, in part, appear hereinafter in the followingdescription of my invention, which is made with reference to theaccompanying drawings wherein:

FIG. 1 is a sectional view of an orbitron pump.

FIGS. 1A and 1B are alternative top Views of the FIG. 1 pumpcorresponding to two alternate modes of construction;

FIG. 2 is a detailed drawing of the anode showing its relation to thefilaments and the shield electrode of the orbitron device.

FIG. 3 is a typical speed-pressure curve showing performance of theimproved device in pumping argon.

Referring now to FIG. 1, the orbitron pump is connected to a vacuumchamber 12, a portion of which is shown in cutaway form, through a spoolpiece 14. Ports in the spool piece provide connections to roughingpumps,

inlet valves and other conventional accessories of a vacuum system.

The pump 10 comprises an outer annular electrode traced by cooling coils22 and capped by a cover flange 24. The cover 24 contains vacuumfeedthroughs with electrical connector pins 31-35, mounted therein.Within the pump body 20 are a glass header 40, which contains sevenmetal pins 41-47, and an anode rod 50. A titanium cylinder 56 is mountedon the anode rod 50. A filament 52 is mounted from the pins 44, 45. Areflector shield 58 is mounted on the three remaining pins (not shown)of header 40. The pins 41, 42, 44, pass through slots 59 in the shield58.

In operation, the outer annular electrode 20 of the pump is grounded anda potential on the order of 10,000 volts positive is impressed on thecenter rod electrode 50. Water is pumped through coils 22'. Thefilaments 52, 54 are heated by an alternating current source (not shown)and biased slightly positive with respect to ground. Electrons arethermionically emitted from the filaments. These electrons orbit aroundthe rod until they collide with a gas molecule in the pump body orstrike the rod 50 or a getter cylinder 56 disposed along the rod 50.Electron-molecule collisions produce positive ions which are attractedto the outer annular electrode 20. Electron bombardment of gettercylinders 56 heats the getter material (titanium) to its vaporizedtemperature and titanium vapors are emitted and condensed on the cooledouter annular electrode 20. The fresh deposits of titanium on the outerannular electrode readily combine with ions or molecules of active gasesand ions of the inert gases by directchemical combination and physicalburial as the titanium vapors continuously stream onto the outer annularelectrode. Inert gases require electron collisions for efficientpumping.

It is desirable to maximize the average trajectory length of theelectrons in the pump body in order to increase their inert gas pumpefficiency. In the present invention this is accomplished by increasingthe distance I from the filaments to the inner electrode 50, by theutilization of smaller dimensions for the cross-sections of the filaments and their supporting lead wires, and decreasing the potential bumpcaused by the presence of the filament and its supports. All these stepsare made possible by the use of thoria-coated iridium filaments. Anadditional benefit is that the design of support pins is simplified, asdescribed below.

The'l spacing is described herein as an optimum fraction of D which isthe minimum cross section dimension of the pump body in a plane passingthrough the central electrode 50. Generally, the orbitron 10 has theform of a cylinder and D is the diameter of outer annular electrode 20as shown in FIG. 1A. But the pump can also have other shapes such as anellipse or a rectangle as shown in the pump 10' of FIG. 1B, where thecharacteristic dimension D is the smallest dimension across the outerelectrode 20 passing through the rod anode 50'. It is also in point tonote that the electrode 50 or 50' (FIG. 1A or 1B, respectively), may bea grid or cylinder contained in a larger pump body. In that case D stillrefers to the dimension across the annular electrode 50 5 or 50'.Dimensions 1 in excess of 7 D are made possible With the use ofthoria-coated iridium filaments. The maximum l is held to less than %Dto avoid excessive collision of the electrons with the electrode 50 or anecessity of high bias on the filament to avoid such collisions.Generally, an I spacing of 4 inch is preferred for fourinch pumps andone inch for six-inch pumps. Throughout the range of positions of I fromabout to D, high emission current can be obtained from thin filaments(up to milliamperes and typically 30 to 40 milliamperes) for highemission and good average trajectory lengths obtained and filamenttemperatures are held very low (about 1400 C. compared to prior artfilament temperatures of about 2700 (3.). Also the filaments 52 and 54may be formed with welded lead connections at the ends of the helix.This allows close spacing of their wire leads (typically inch comparedto about 4 inch in prior art devices) and spacing between the filament(e.g. 52) and the shield lead (e.g. 42) can be held very close between Aand inch), thus limiting the disturbance caused by the filamentstructure in the orbitrons electrostatic field. The desirable range offilament to shield pin spacing may be expressed as between one and twohelix diameters.

Referring to FIG. 2, there is shown a preferred form of anode, forutilizing the orbitron device as a high speed six-inch pump. The rod 50is .08" diameter tungsten. The titanium cylinders 560 and 56F are 7diameter by 1" long and separated by a ,4 spacing. The cylinders 56N and56M are diameter by 1" long and also apart. The letter dimensions are asfollows (in inches):

a .250 b .375 c 3,250 d 4.0 e 1.0 Radius l-1.0

Springs 57 are used to hold the titanium cylinders in place. Thefilament helices were /3 in length. The filaments 52, 54 were made of.005" diameter thoria-coated iridium wire made as described in Redhead;Can. J. Physics, vol. 40: p. 1814 (1962) and Hanley, Journal of AppliedPhysics, vol. 19: p. 583 (1948) and coiled in a helix diameter of .030"with a pitch of 16 turns per inch.

FIG. 3 shows a typical performance curve for a (nominal) six-inch pumpconstructed as described above. The abscissa of the curve is pressure intorr and the ordinate is argon pumping speed in liters per second. Thespeed of 21-22 liters per second at low pressure is very high for thissize pump in comparison to prior art devices. The same pump was testedfor air speed and pumped air at speeds in excess of 1,000 liters persecond at low pressure (below l torr). The argon speed tests wereconducted with an anode potential of 10 kilovolts and filament emissioncurrent of 50 milliamperes and filament bias of 120 volts positive. Theair speed tests were conducted with the anode at 10 kilovolts, filamentemission current -50 ma. and filament bias -l30 volts positive. Bothfilaments 52 and 54 were operated at temperatures of about l300 C. to1400 C.

These significant improvements in performance are due to the use of thethoria-coated iridium filaments. The l spacing of one inch would not befeasible with the conventional tungsten filaments. The large I spacinggives a high electron average trajectory length for ionization of gasmolecules and this mechanism accounts for substantially all the pumpingof argon.

While I have described my improved orbitron device in terms of its usageas a pump, usage of the device is also possible as a gauge, amplifier,or in other forms, utilizing the features of the present invention. Itis therefore intended that the above description shall be read asillustrative and not in a limiting sense.

What is claimed is:

1. An orbitron device comprising, in combination,

(a) an outer annular electrode,

(b) a straight inner electrode located centrally within said outerelectrode so that a space is formed between the inner and outerelectrodes, said inner and outer electrodes occuping a length in excessof the minimum cross section dimension of said outer electrode,

(c) means for producing a positive high voltage potential on said innerelectrode,

((1) at least one electron emitting filament made of thoria-coatediridium and located at an end of the axial space between electrodes andat a radial distance from said inner electrode of between about and theminimum cross section dimension of said outer electrode, the filamentbeing essentially parallel to the inner electrode, and

(e) means for shielding the filament from the inner electrode.

2. The device of claim 1 wherein the filament has the form of a helixwith its axis parallel to the inner electrode, the helix being supportedon a pair of lead pins with one of said pins constructed and arranged assaid shielding means.

3. The device of claim 2 with said pins being parallel over a portion oftheir lengths with a spacing, the coil diameter of the filament beingless than one pin diameter.

4. The device of claim 2 with the helix filament being essentiallyparallel to its shield pin and having a spacing therefrom of between oneand two helix diameters.

5. An orbitron vacuum pump comprising, in combination,

(a) an outer annular electrode,

(b) a straight inner electrode located centrally within said outerelectrode so that a space is formed between the inner and outerelectrodes, said inner and outer electrodes occuping a length in excessof the minimum cross section dimension of said outer electrode, theinner electrode having getter material mounted thereon,

(c) means for producing a positive high voltage potential on said innerelectrode,

(d) at least one electron emitting filament made of thoria-coatediridium and coiled into a straight helix which is located at an end ofthe axial space between electrodes and at a radial distance from saidinner electrode of between about and the minimum cross section dimensionof said outer electrode, the filament helix being essentially parallelto the inner electrode, and

(e) a pair of lead pins supporting the helix filament at its ends, oneof said pins being disposed between the filament and inner electrode andparallel thereto to serve as a shield, the diameter of said shield pinbeing greater than the helix diameter, and the shield pin and helixhaving a spacing therebetween of from one to two helix diameters, thelead pins being parallel along the portion thereof axially beyond thefilament and having a spacing of about three pin diameters.

References Cited UNITED STATES PATENTS ROBERT M. WALKER, PrimaryExaminer.

1. AN ORBITRON DEVICE COMPRISING, IN COMBINATION, (A) AN OUTER ANNULARELECTRODE, (B) A STRAIGHT INNER ELECTRODE LOCATED CENTRALLY WITHIN SAIDOUTER ELECTRODE SO THAT A SPACE IS FORMED BETWEEN THE INNER AND OUTERELECTRODES, SAID INNER AND OUTER ELECTRODES OCCUPING A LENGTH IN EXCESSOF THE MINIMUM CROSS SECTION DIMENSION OF SAID OUTER ELECTRODE, (C)MEANS FOR PRODUCING A POSITIVE HIGH VOLTAGE POTENTIAL ON SAID INNERELECTRODE, (D) AT LEAST ONE ELECTRON EMITTING FILAMENT MADE OFTHORIA-COATED IRIDIUM AND LOCATED AT AN END OF THE AXIAL SPACE BETWEENELECTRODES AND AT A RADIAL DISTANCE FROM SAID INNER ELECTRODE OF BETWEENABOUT 3/16 AND 3/8 THE MINIMUM CROSS SECTION DIMENSION OF SAID OUTERELECTRODE, THE FILAMENT BEING ESSENTIALLY PARALLEL TO THE INNERELECTRODE, AND